4 Answers
4

Butter is at most 82% oil and the rest is water. So, you're going to need to add more butter for the same amount of oil. Besides, you need to adjust the liquid content accordingly since you are going to have more water than you planned.

If you are going to use the same amount of oil, then it will not make any difference other than the taste. Just make sure they are of same state (i.e. melted butter instead of vegetable oil or vice versa)

In practice, for most recipes, the water content in butter is small enough that the variance is well within the tolerance of the recipe; in practice, oil and butter can be substituted 1:1 in quick breads like muffins.
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SAJ14SAJJun 2 '13 at 9:36

If you're making something like muffins, then in my experience the water content is a non-issue. Yes, there's some water. But muffin batter has such a broad range of tolerance that as long as you're basically familiar with the target consistency it'll be fine.

That said, the amount of oil or butter that goes into a batch of muffins is not really enough to impart much butter flavor.